The present invention is directed to an information storage medium and the method for recording and retrieving information thereon.
Optical disk digital record/playback systems wherein digital information is stored (recorded) and played back (retrieved) from a disk shaped storage medium are known in the art. Such systems are important in meeting the need for a digital mass memory system that is capable of storing large numbers of bits of information, for example 10.sup.13 bits, with very high data transfer rates such as 400 megabits per second. Numerous attempts have been made to produce a memory system having such capacity and speed but without success. Generally, the approaches to designing such systems have been limited to inducing reflective or transmissive changes in the optical recording medium. Laser beams are generally used for recording information on the disk, and reflected laser light is generally used for retrieving the information.
Various systems have been proposed for recording information on photosensitive materials. For example, U.S. Pat. No. 4,128,897 issued Dec. 5, 1978 to Norton et al discloses a photosensitive storage medium comprising a doped semiconductor p-type substrate covered by an n-type semiconductor layer. The n-type semiconductor layer is in turn covered by a non-doped layer. The doped and non-doped layers must be capable of eutectic formation. The non-doped layer is biased with respect to the substrate so as to produce a back-biased junction between the p- and n-type semiconductor sandwich. Information is recorded on the medium by a focused beam, either laser or electron, which heats the non-doped layer and the n-type semiconductor layer to form a eutectic alloy between the two. Information is retrieved from the medium by applying a focused electron beam so as to produce high current flow due to hole migration at the unalloyed sites and low current flow at the alloyed sites.
U.S. Pat. No. 3,902,010 issued Aug. 26, 1975 to Goshima also discloses a photosensitive storage medium. The medium comprises a glass substrate covered by a highly photosensitive chalcogen glass. The highly photosensitive glass is in turn covered by a chalcogen glass having a lesser photosensitivity. The chalcogen layers are separated by a metal film. The high sensitivity chalcogen layer is separated from the substrate by another metal film. Two laser beams, one steady and one modulated are required. The modulated beam is flanked by the steady beam. The modulated beam sensitizes the highly photosensitive chalcogen glass but not the lower sensitivity chalcogen glass. The steady beam sensitizes both layers of chalcogen glass and produces two diffused regions flanking the region irradiated by the modulated beam. Metal is diffused into the chalcogen glass so as to form a region insensitive to alkali. Post-treatment is required wherein the medium is treated with an alkaline liquid so as to remove the unexposed portions of the chalcogen glass. Alternatively, the exposed portions of the chalcogen glass may be removed by post-treatment methods.
U.S. Pat. No. 3,865,975 issued Feb. 11, 1975 to Fletcher et al discloses a photosensitive storage medium comprising a photoconductive semi-conductor film with high concentrations of hole and electron traps, both of which are deep traps. The photoconductive semiconductor film is sandwiched between transparent conductive plates which are electrically biased. To record information, energy is applied in excess of the energy gap between the conductive and valence bands. To retrieve information, energy is applied at a level below the band gap but in excess of the energy band of the trapped electrons and/or the trapped holes so as to de-trap the electrons and holes and excite the electrons and holes into the conduction band and valence band.
A system for recording information on a storage medium by means of radiation damage has also been proposed by Munakata and Miyazaki, Japanese Journal of Applied Physics, Vol. 20, No. 4, April, 1981, pp. L293-L295. In this system, the storage medium comprises a doped p-type Si wafer having a SiO.sub.2 film cover. The medium does not employ a p-n junction. An electron beam produces radiation damage in the p-type layer. Other particle beams, such as a X-ray beam, are also proposed for introducing the radiation damage. The irradiated areas of the medium are less conductive than the non-irradiated areas. Information is retrieved by applying a chopped photon beam to the medium and detecting differences in the surface photovoltage. Information may be erased from the medium by heating the medium so as to remove the radiation damage.
To my knowledge, no one has heretofore proposed the use of a non-annealed semiconductor doped with a first impurity and ion implanted with a second impurity for the recordal and retrieval of digital information. In particular, it has not been proposed to record information on such a medium by selectively annealing discrete, localized regions of the medium. By annealing is meant the application of energy to re-order displaced atoms in the medium. Such atom displacement or lattice defects are known to occur as a result of ion implantation.